Electrical interference reducer for gating apparatus of an electro multiplier

ABSTRACT

An apparatus for eliminating noise at the oscilloscope output of a magnetic electron multiplier by arranging the output anodes of the gating section so that the oscilloscope output anode is located on a different axis than the other output anodes and is protected from electromagnetic interference by a shield located between the oscilloscope anode and the other output anodes. The oscilloscope anode is preferably cup-shaped to prevent the escape of electrons impinging the anode.

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[54] ELECTEHCAL HNTEREERENCE REDUCER FOR GATING APPARATUS OF AN ELECTRON MULTIPLIER [72] lnventors: Donald C. Damoth, Rochester, N.Y.; William 1-1. Shriner, Blanchester, Ohio [73] Assignee: The Bendix Corporation [22] Filed: Nov. 7, 1969 [21] App1.No.: 874,830

[52] 11.5. C1. .313/237, 250/419 D, 313/103, 313/105, 313/239 [51] llnt. Cl. .1101] 1/00 [58] lField of Search ..313/l03-105, 237, 313/239; 250/419 D [56] References Cited UNITED STATES PATENTS 2,197,652 4/1940 Statz et a1. ..3l3/l03 2,674,661 4/1954 Law ..313/l05 X Cohen et al.... Gielow et a1.

14 Feb. 22,

3,474,276 10/1969. Betoule et a1. ..313/103 X OTHER PUBLICATIONS Clark, George L., The Encyclopedia of Spectroscopy Reinhold Publishing Corporation, New York, Received December 23,1965, QC 451 C55, page 635.

Primary Examiner-David Schonberg Assistant ExaminerToby H. Kusmer Attorney--Raymond J. Hitler and Flame, Arens, Hartz, Smith and Thompson STRACT An apparatus for eliminating noise at the oscilloscope output of a magnetic electron multiplier by arranging the output anodes of the gating section so that the oscilloscope output anode is located on a different axis than the other output anodes and is protected from electromagnetic interference by a shield located between the oscilloscope anode and the other output anodes. The oscilloscope anode is preferably cupshaped to prevent the escape of electrons impinging the anode.

3 Claims, 2 Drawing Figures 4 ELECTRICAL INTERFERENCE REDUCIER FOR GATING APPARATUS OF AN ELECTRON MULTIPLIER BACKGROUND OF THE INVENTION This invention relates to an improved gating apparatus for an electron multiplier of a mass spectrometer and. more specifically to a collecting anode of the gating apparatus which transmits an output signal to an oscilloscope.

The mass spectrometer is an instrument that permits rapid analysis of molecular species by measurement of the masses of the different ions after ionization of the molecules. In operation, a small amount of gas to be analyzed is admitted through a sample inlet into an ionization chamber or region where the gas is ionized by electrons emitted from a filament. The ions are then directed or accelerated by an electric field from the ionization chamber and into a region where the ions are separated according to their mass to charge ratio (m/e). The ions then impinge upon the cathode of an electron multiplier to achieve a gain of 10 or greater. The resulting output signal is then synchronized on an oscilloscope and/or gated to an analog for strip chart recording to indicate the mass spectrum of the gas under analysis. Multipliers having more than one gate give the spectrometer a built-in capability to monitor multiple mass peaks of the spectrum simultaneously by the addition of analog scanners. Each scanner is capable of scanning the mass range from to 750 atomic mass units, or any portion thereof. The Bendix Corporation Time of Flight Mass Spectrometer, Model 3012, is one mass spectrometer which allows up to six analog scanner plug-in units to be used simultaneously with the oscilloscope output.

However, electron multipliers having more than one gate produce noise which obscures some of the mass spectrum lines displayed on the oscilloscope. Generally, the noise is picked up by the oscilloscope anode from the voltage pulses (e.g. 100 volts pulses of nanoseconds duration) applied to the gating electrodes associated with the analog outputs and appears on the oscilloscope display as a ringing or oscillatory signal superimposed on the mass spectrum. Although the noise decays expontentially after the application of each gating pulse, it persists in sufficient magnitude for up to 100 nanoseconds so that approximately 10 mass peaks on the oscilloscope display are obscured. It follows that when more than one analog channel receives gating pulses the noise at the oscilloscope anode is increased so that a substantial portion of the mass spectrum is obscured. This reduces the value of the oscilloscope display.

SUMMARY OF THE INVENTION To improve the resolution of the mass spectrum lines displayed on an oscilloscope, electrical interference at the oscilloscope anode of the electron multipliers gating apparatus is reduced by separating and shielding the oscilloscope anode from the other anodes of the gating apparatus. The invention is characterized by a gating apparatus having the oscilloscope anode electrically shielded from the other anodes and on an axis coincident or parallel to the direction of the cycloidal path of electrons leaving the electron multiplier. The oscilloscope anode is characterized by the fact that it is shaped (preferably like a cup) to retain substantially all the electrons which impinge the anode. Further, since the oscilloscope anode incurs the most continuous use of all the anodes of the gating apparatus, the oscilloscope anode is demountably attached to the gating apparatus so that it may be removed for cleaning and replaced without disassembly of the gating apparatus. The invention is especially useful in improving the performance of the type of gating apparatus shown in U.S. Pat. No. 3,049,638.

Accordingly, it is an object of this invention to reduce the electrical interference picked up by the oscilloscope anode of an electron gating apparatus.

It is another object of this invention to provide an improved gating apparatus for electron multipliers.

It is still another object of this invention to improve the performance of mass spectrometers.

It is a further object of this invention to provide an improved gating apparatus which permits removal of the oscilloscope anode for cleaning without disassembly of the gating apparatus.

The above and other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings and claims which form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a diagrammatic view of a mass spectrometer utilizing the invention.

FIG. 2 is a more detailed partialplan view of the gating apparatus shown in FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawings, FIG. I shows a mass spectrometer of the type described in U.S. Pat. No. 2,765,408 and 2,685,035. Molecular species entering the ionizing region I from the sample inlet 3 are ionized by electrons from filament 5. The ions 10 are then accelerated into the drift tube 7 by accelerating grids 9. Because of the length of the drift tube 7 and the different velocities of the ions, the ions are separated according to their mass to charge ratio (m/e) striking the cathode 20 at different times. The different times of flight (I) for ions may be calculated mathematically from the equation T=k (m/e)", where k is a constant depending on physical dimensions. For example, the time of flight of a singly charged nitrogen ion m 28 atomic mass units in the Bendix Time of Flight Mass Spectrometer is 5 microseconds and under usual conditions the time width of the nitrogen pulse at the spectrometers output is about 0.015 microsecond (about 0.060 microsecond for atomic species of 400 AMU). The separated ions pass through aperture 2 of grid 4 and strike the cathode 20 to produce secondary electrons 40. A magnetic electron multiplier 30 (e.g., U.S. Pat. Nos. 3,049,638 and 2,841,741 is used to detect and amplify the ions striking the cathode 20. The cathode 20 may be disposed at an acute angle with a plane that is perpendicular to the longitudinal axis 8 of the drift tube 7 to decrease the time interval for all the electrons, propogated from the cathode by ions of the same mass, to strike the dynode. The electrons emitted from the cathode then follow a cycloidal path 41 under the influence of the mutually perpendicular electric and magnetic fields in the multiplier striking the dynode strip 31 and multiplying in number to achieve a gain of approximately 10 The resulting output signal is then synchronized on an oscilloscope, and/or gated (gating apparatus 35) to an analog (not shown) for strip chart recording. The output to the oscilloscope (anode 63) is located at the end of the gating apparatus 35, and is in the path 41 of the electrons traveling from between the dynode and the field strip 32. The anode 63 requires no gating signal and receives all the electrons not gated to one of the analog anodes of the apparatus 35. A shield 39 is located between the oscilloscope output and the other output gates to protect the anode 63 from electrical interference.

Plates 70 and arms form a matrix that precisely locates, with respect to each other, the gating assembly 35, the field strip 32, the dynode 31 and the rail 50. The matrix may also be used at the input to the electron multiplier 30 to precisely angle the cathode 20. At the input, the arms 80 are placed in the holes 72 having predetermined locations so that when the arms 80 are placed in the holes 72, the cathode 20, held by the arms 80, is held in a position relative to the location of the holes 72.

FIG. 2 shows a gating assembly 35, of the type shown in US. Pat. No. 3,049,638, which receives electrons from the electron multiplier 30. (FIG. 1). Mounting Plate 70 has a plurality of arms 80 which form a matrix assembly that precisely arranges the gating components. Located on axis B is the oscilloscope anode 63 which receives electrons not gated by one of the gating electrodes 54 to one of the output anodes 62 on axis A. A shield 39 protects the oscilloscope anode 63 from receiving stray electrons and electromagnetic interference generally caused by the potentials applied to the gating electrodes 54. The source of stray electrons are the electrons which were gated to an output anode 62 on axis A but have strayed from their course. The oscilloscope anode is shaped so that electrons impinging the anode do not escape. Preferably, the anode is cup-shaped so as to form a container which retains substantially all the electrons entering the container. For example, the oscilloscope anode may be comprised ofa base and a wall disposed around at least a portion of the periphery of the base and extending away from the base so as to form a container which retains substantially all the electrons entering the container. Further, since the oscilloscope anode 63 receives the most use, it is desirable that the oscilloscope anode 63 be frequently cleaned. Therefore, the oscilloscope anode 63 is demountably attached to the mounting plate 70 so that it may be removed for cleaning.

The gating assembly 35 includes the rail 50 which is aligned with the dynode of the electron multiplier. The magnetic field of the electron multiplier extends through the rail 50 so that the cycloidal path of the electrons continues, but is changed a predetermined amount by an electric field between the rail 50 and gates, so that electrons do not strike rail 50. In the absence of a gating pulse the electrons travel in a cycloidal path close to or along axis B until they strike the oscilloscope anode 63. Precisely spaced from the rail 50 and apart from the anode 63 is the gating wall 52. Adjacent the gating wall 52 is a plurality of gating electrodes 54 each of which is precisely spaced from rail 50 and each of which has a transverse rod 56 secured (preferably welded) to one end thereof. Connected to each ofthe gating electrodes 54 is a pulser (not shown) which maintains the electrodes 54 at zero potential until it is desired to divert the path of the electron cycloid 41 to a collecting anode 62 by applying a negative potential (preferably a negative voltage greater than that applied to the rail 50, such as 70 volts) to the appropriate gating electrode 54. Precisely spaced from each electrode 54 is an anode plate 60 which is maintained at a constant voltage (preferably a negative voltage greater than that applied to the collecting anode, such as l volts). Adjacent to each anode plate 60 is the collecting anode 62 which is connected to an output device (not shown). A recorder or other indicating and/or actuating device may be connected to output A, B. C and/or D. The anode plates 60 are preferably spaced from collecting anodes 62 a precise distance, which is less than the cycloid height, to provide sufficient space for the electrons, diverted by a negative pulse to the proper electrode 54, to strike the collecting anode 62. The gating electrodes 54 are precisely spaced from the anode plates 60, collecting anodes 62 and rail 50 to establish the proper electric field therebetween. A power supply and pulser (not shown) supply the voltages to the gating components to establish at predetermined time intervals the electric fields necessary to direct electrons to the proper output. Although 4 outputs are shown, more or less may be used as desired.

OPERATION In describing how the location and configuration of the oscilloscope anode improves the performance of the gating apparatus 35, reference is made to FIG. 2 and the cycloidal path 41 ofthe electrons as they travel between the rail 50 and gating electrodes 54. The cycloid is formed through the cooperation of the electrical and magnetic field existing between and beyond the field strip of the electron multiplier (not shown). The cycloid moves in a direction toward oscilloscope anode 63 and by the time the electrons reach the rail 50, they are traveling in a cycloidal path 41 that has been elevated a predetermined amount by the application of a proper electric field. To maintain a uniform electric field so that the electron cycloid travels parallel to rail 50 the gating electrodes are exactly spaced from the rail. The electrons continue to travel in a cycloidal path parallel to the rail 50 until one of the gating electrodes is pulsed in an appropriate manner or until they strike oscilloscope anode 63. or example, when gating electrode'54 of output D receives a negative voltage greater than the voltage applied to the rail 50, the cycloid is diverted towards gating anode 62 at output D. An electric field between the gating anode 62 and anode plate 60 then directs the electrons towards the collecting anode 62. A shield 39 disposed between anode 63 and the gating electrodes 54 isolates anode 63 from electrical interference as a result of pulses applied to the gating electrodes 54. When electrons are not diverted by a pulse applied to a gating electrode 54, they continue along their cycloidal path to the oscilloscope anode 63. Because ofthe shape of the oscilloscope anode 63, electrons impinging upon its surface are retained. This increases the current collecting ability of the oscilloscope anode which increases signal strength at the anode (signal-tonoise ratio at oscilloscope output is increased).

While a preferred embodiment of the invention has been disclosed, it will be apparent to those skilled in the art that changes may be made to the invention as set forth in the appended claims, and, in some cases, certain features of the in vention may be used to advantage with corresponding use of other features. Accordingly, it is intended that the illustrative and descriptive materials herein be used to illustrate the principles of the invention and not to limit the scope thereof.

Having described the invention, what is claimed is:

I. In combination with a magnetic electron multiplier of the type having a rail and a first anode for collecting electrons located opposite said rail, said first anode located on a first axis, said first axis being parallel to said rail, the improvement comprising.

a second anode for collecting electrons located adjacent said rail said first anode on a second axis between said rail and said first axis, said second axis generally parallel to said rail and said first axis so that electrons diverted to the first anode do not strike said second anode and electrons not diverted to the first anode strike said second anode, said second anode comprising a base and a wall disposed around at least a portion of the periphery of said base so as to retain substantially all electrons that strike said base; and

means for shielding said second anode from electromagnetic interference.

2. The combination as recited in claim 1 wherein said means for shielding said second anode from electromagnetic interference comprises: a plate located between said second anode located on said second axis and said first anode located in said first axis to protect said second anode from receiving stray electrons when electrons are diverted to said first anode.

3. The combination as recited in claim 2 wherein said second anode is demountably attached to said gating apparatus. 

1. In combination with a magnetic electron multiplier of the type having a rail and a first anode for collecting electrons located opposite said rail, said first anode located on a first axis, said first axis being parallel to said rail, the improvement comprising. a second anode for collecting electrons located adjacent said rail said first anode on a second axis between said rail and said first axis, said second axis generally parallel to said rail and said first axis so that elEctrons diverted to the first anode do not strike said second anode and electrons not diverted to the first anode strike said second anode, said second anode comprising a base and a wall disposed around at least a portion of the periphery of said base so as to retain substantially all electrons that strike said base; and means for shielding said second anode from electromagnetic interference.
 2. The combination as recited in claim 1 wherein said means for shielding said second anode from electromagnetic interference comprises: a plate located between said second anode located on said second axis and said first anode located in said first axis to protect said second anode from receiving stray electrons when electrons are diverted to said first anode.
 3. The combination as recited in claim 2 wherein said second anode is demountably attached to said gating apparatus. 